Method and system for optimized LNG production

ABSTRACT

A method and system for producing liquefied and sub-cooled natural gas by means of a refrigeration assembly using a single phase gaseous refrigerant comprising: at least two expanders ( 1 - 3 ); a compressor assembly ( 5 - 7 ); a heat exchanger assembly ( 8 ) for heat absorption from natural gas; and a heat rejection assembly ( 10 - 12 ). The novel features according to the present invention are arranging the expanders ( 1 - 3 ) in expander loops; using only one and the same refrigerant in all loops; passing an expanded refrigerant flow from the respective expander into the heat exchanger assembly ( 8 ), each being at a mass flow and temperature level adapted to de-superheating, condensation or cooling of dense phase and/or sub-cooling of natural gas; and serving the refrigerant to the respective expander in a compressed flow by means of the compressor assembly having compressors or compressor stages enabling adapted inlet and outlet pressures for the respective expander.

BACKGROUND OF THE INVENTION

The energy demand in the world is increasing, and the forecast is acontinued growth. Gas as an energy carrier has received increasedattention recent years, and it is predicted that gas will become evenmore important. In order to transport gas over longer distances,liquefied natural gas, LNG, is often regarded as the best option,especially overseas.

Stranded gas or associated gas are gas sources which are “wasteproducts” from oil production. These gas sources are today seldomutilized. They are commonly flared. With the increasing gas prices andmore focus on the environment, it has become more economically viableand more politically important to utilize these sources. Many of thesesources are offshore, and liquefaction on a floating production storageand offloading, FPSO, unit is in many cases the best option. FPSO'soffer flexibility since they can be moved relatively easy to othersources. A challenge on the FPSO's is the space available. Furthermore,the weight of the equipment should be minimized, and the refrigerantshould preferably be non-combustible.

An important issue for LNG production is the energy demand. High energydemand per kg produced LNG, i.e. specific energy consumption, makes itless profitable and less environmental friendly. The number ofeconomically viable gas sources will be narrowed. Besides reducingoperating cost, lower specific energy demand will also save investmentcost, since the equipment will be smaller.

LNG production onshore does not have the same limitations with regard toweight and space but energy efficient LNG production is just asimportant. As the capacities of the plants gets larger, energyefficiency becomes more important.

Technology involving multi component refrigerant, MCR, often in cascadesarrangements, is regarded as the most efficient technology for LNGproduction. It is commonly used in larger plants, base load plants, andto some extent in medium scale plants. Due to its complexity,MCR-technology is costly and control is slow. In addition, a gas make-upassembly is needed to ensure the correct composition of the MCRrefrigerant. Another disadvantage is that the refrigerant is combustiblewhich may be a problem, especially in offshore installations.

If a single component refrigeration technology using an inert gas, suchas nitrogen, can be comparably energy efficient, it will represent amajor improvement in terms of cost, compactness, weight, robustness,control, and safety. This technology can then be interesting toimplement also in large scale plants.

U.S. Pat. Nos. 5,768,912 and 5,916,260 propose processes for LNGproduction based on nitrogen single refrigerant technology. Therefrigerant is divided into at least two separate flows which are cooledand expanded in at least two separate expanders. Each of the flows areexpanded down to the suction pressure of the compressor train, which isthe lowest refrigerant pressure in the arrangement, thus using moreenergy than necessary.

U.S. Pat. No. 6,412,302 describes a LNG liquefaction assembly using twoindependent expander refrigeration cycles, one with methane or a mixtureof hydrocarbons, and the other with nitrogen. Each cycle has oneexpander operating at different temperature levels. Each of the cyclescan be controlled separately. Using two separate refrigerants willrequire two refrigerant buffer systems. Also using a flammablerefrigerant implies restrictions or extra equipment.

Several patents are granted for MCR processes and apparatus usingprocess gas as refrigerant, e.g. U.S. Pat. No. 7,225,636 and EP patent1455152. Common for these are that heat absorption includes phase changeof refrigerant, which inherently gives a more complex system. Moreequipment is needed and the control becomes complicated and sensitive.

There is a need for efficient processes based on an inert singlecomponent refrigerant. The present invention describes an energyefficient and compact LNG production assembly with a flexible controlusing an inert gas as refrigerant.

SUMMARY OF THE INVENTION

The current invention relates to a method and apparatus for optimizedproduction of LNG. In order to minimize the specific energy consumption,the heat exchanger losses have to be minimized. This is achieved byarranging at least two expanders in single component and single phaserefrigeration cycle(s) so that the mass flows, temperatures and pressurelevels into the expanders can be controlled separately. By thisarrangement, the refrigeration process can be adapted to varying gascompositions at different pressures and temperatures, and at the sametime optimize efficiency. The control is inherently robust and flexible.A LNG production plant according to the present invention can be adaptedto different gas sources and at the same time maintain the low specificenergy consumption.

In one aspect the present invention relates to a method for producingliquefied and sub-cooled natural gas by means of a refrigerationassembly using a single phase gaseous refrigerant comprising: at leasttwo expanders; a compressor assembly; a heat exchanger assembly for heatabsorption from natural gas; and a heat rejection assembly, and furthercomprising: arranging the expanders in expander loops; using only oneand the same refrigerant in all loops; passing an expanded refrigerantflow from the respective expander into the heat exchanger assembly, eachbeing at a mass flow and temperature level adapted to de-superheating,condensation or cooling of dense phase and/or sub-cooling of naturalgas; and serving the refrigerant to the respective expander in acompressed flow by means of the compressor assembly having compressorsor compressor stages enabling adapted inlet and outlet pressures for therespective expander.

In another aspect the present invention relates to a system forproducing liquefied and sub-cooled natural gas by means of arefrigeration assembly using a single phase gaseous refrigerantcomprising: at least two expanders; a compressor assembly; a heatexchanger assembly for heat absorption from natural gas; and a heatrejection assembly, wherein the expanders are arranged in expanderloops; only one and the same refrigerant is used in all loops; anexpanded refrigerant flow from the respective expander is passed intothe heat exchanger assembly, each being at a mass flow and temperaturelevel adapted to de-superheating, condensation or cooling of dense phaseand/or sub-cooling of natural gas; and the refrigerant to the respectiveexpander is served in a compressed flow by means of the compressorassembly having compressors or compressor stages enabling adapted inletand outlet pressures for the respective expander.

Favourable embodiments are specified by the dependent claims.

Outlet pressures of the expanders are controlled to be as high aspossible but at the same time feeding the heat exchanger arrangement forsub-cooled LNG production with required refrigerant temperatures.Suction pressures for each of the compressor stages are then kept ashigh as possible. This is unlike prior art, see e.g. U.S. Pat. No.5,916,260, wherein all streams are expanded down to the lowestrefrigerant pressure. A major improvement with the present invention isthat specific work and suction volumes of the compressors are minimized,thus improving the overall system efficiency. Pipeline dimensions arereduced with smaller valves and actuators as a consequence. All thesefactors contribute to a significant cost and space need reduction.Installation work will also become less complicated and hence moreefficient.

Reducing heat exchanger losses is of vital importance in low temperatureprocesses. An important embodiment of the present invention is that itreduces the temperature differences to a minimum by adapting therefrigeration process to the principally three different stages of LNGproduction: de-superheating, condensation (cooling of dense phase atsupercritical pressures) and sub-cooling. This is unlike prior arttechnology, e.g. U.S. Pat. No. 6,412,302, not having separate adaptationfor de-superheating and condensation/cooling of dense phase.

The present invention will operate with single refrigerant in the gasphase. Nitrogen is an obvious alternative. The non-flammability isregarded as an advantage in for instance offshore installations. Usingonly one single component refrigerant also reduces complexibility.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate preferred embodiments of thepresent invention.

FIG. 1 shows the principle stages of liquefied natural gas productionwith corresponding cooling capacity needs represented by the straightlines.

FIG. 2. illustrates an example of the warm and cold composite curves ofthe present invention.

FIG. 3 depicts an embodiment of the present invention including threeexpanders.

FIG. 4 shows another embodiment including three expanders arranged inthree separate refrigeration cycles.

FIG. 5 illustrates an embodiment only including two expanders.

FIG. 6 depicts an embodiment like FIG. 5 but with expanders arranged inseparate refrigeration cycles.

FIG. 7 shows an embodiment allowing for splitting and mergingrefrigerant streams.

FIG. 8 illustrates a section of FIG. 7 in which at least one of theexpanders illustrated in FIGS. 3 to 6 is provided with expanders coupledin series.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to production of liquefied natural gas,LNG. Dependent on the gas source, the composition will vary. Forinstance, a gas composition can include 88% methane, 9% heavierhydrocarbons, 2% carbon dioxide, and 1% water, nitrogen and other tracegases. Before liquefaction, the concentration of carbon dioxide, water(which will freeze) and harmful trace gases such as H₂S needs to bereduced to acceptable levels or eliminated from the gas stream. The wellgas will undergo a pre-treatment step before entering the liquefactionstep. In FIGS. 3 to 6, this pre-treated natural gas stream is indicatedwith reference numeral 9.

The process of LNG production can principally be divided into threedifferent stages. A) De-Superheating, B) Condensation and C)Sub-cooling, see the schematically sketch in FIG. 1. The criticalpressure of methane is around 46 bar. Dependent on the natural gassource composition, the critical pressure will vary from 46 bar andupwards. Above critical pressure for a natural gas composition,condensation is not possible. However, instead of condensation, the gaswill pass a stage with increased specific heat capacity.

Each of the stages requires different specific cooling capacity. Inorder to reduce heat exchanger losses, the temperature differencesbetween warm flows and cold flows in the whole LNG production processhave to be minimized. By utilizing a multiple of expanders, where eachof them can be controlled separately with mass flow, pressure levels andtemperatures, it is possible to achieve a close temperature adaptationbetween refrigeration capacity and the cooling need. Cooling capacitiesfor the three stages are in FIG. 1 represented by three straight lines.Independently controlled expanders give the main contribution to thecooling capacity at each stage. The optimum number of expanders willdepend on the gas source composition, gas pressure, requiredtemperatures and the capacity of the LNG plant.

FIG. 3 shows a configuration according to the present invention. Threeexpanders 1, 2, 3, e.g. turbo expanders, supply a cold box 8 withexpanded gas flows at different temperatures adapted to the liquefactionprocess of the natural gas flow 9. A compressor train 5, 6, 7 serves allthree expanders. The expander 3 supplies the cold box 8 with a flow 60adapted to perform an efficient sub-cooling of the natural gas flow 9,for instance with a temperature interval from −85° C. down to −160° C.,see FIG. 1. Above −85° C., the flow 60 contribute with limited netrefrigeration capacity in the cold box 8, since a mass flow 59 and massflow 61 supplied and returned by the expander 3, respectively, areequal. The expander 2 supplies the cold box 8 with a flow 56 adapted toperform the condensation or cooling of gas at high heating capacity, seeFIG. 1. This process may have a temperature interval between −85° C. and−25° C. Analogous to the expander 3, the mass flow 55 and mass flow 57supplied and returned by expander 2, respectively, will have limitedcontribution to the cooling capacity above −25° C. The expander 1 servesthe cold box 8 with a flow 52 adapted to perform the de-superheatingfrom an inlet temperature of the natural gas flow 9, down to the upperworking temperature of the expander 2, i.e. −25° C. Supplied andreturned mass flows are represented by reference numerals 51, 53.

The compressors 5, 6, 7 are mounted in series forming a compressortrain. The compressor train may consist of various number of stages andone or more compressors in parallel at each stage. The pressure ratiosover each stage are optimized to the temperature requirements in thecold box 8. These pressure ratios and mass flows may be varied andcontrolled during operation by speed control of the compressors.Capacities and temperature ranges can then be adjusted.

By varying the total inventory in the arrangement, the overall pressurelevels can be varied and overall capacity controlled. An inventorybuffer assembly is connected to the suction side of the low pressurecompressor stage, and to the discharge side of the high pressurecompressor. The valves 32 and 34 are used for control of refrigeranttransmission to the buffer tank 25.

Heat is rejected to the ambient by heat exchangers 10, 11,12.

FIG. 3 also shows an example on how the different expanders 1, 2, 3 areconnected to the compressor train 5, 6, 7. The expander 3 is fed byoutlet gas, flow 58, from a heat rejection heat exchanger 11, whereasthe other two expanders 1, 2 are fed by outlet gas, flow 50, 54, fromthe heat rejection heat exchanger 10. Generally, expander inlet andoutlet pressures can be adapted to each expander by applying the presentinvention.

The embodiment according to FIG. 3 illustrates that the cold box 8 isserved by three separate expander loops. Due to for instance mechanicalrequirements for the cold box assembly 8, it may be advantageous tosplit and merge refrigerant flows in connection with the cold boxassembly 8. FIG. 7 shows an example for the splitting and merging ofrefrigerant flows. The warm flow 50 is split into flow 51 and flow 55upstream of the expanders. The cold flows 52 and 56 are mergeddownstream of the expanders into flow 54. By splitting the warm flowsupstream of the expanders, and merging the cold flows downstream of theexpanders, an efficient process can be achieved. However, thisconfiguration has the inherently disadvantage that individual inlet andoutlet pressure adaptation for each expander is not possible. Thepotential for optimized energy efficiency is reduced.

By applying this embodiment, all of the compressors and expanders areintegrated in the same refrigeration arrangement. This gives thepotential to make a very compact solution for the rotating equipment,thus reducing cost. Furthermore, each of the compressor stages 5, 6, 7suck from three different suction pressures, which are formed by theexpanders 1, 2, 3. By suction from highest possible pressures, i.e. massflows 61, 57, 53, the compressor work is minimized, improving theoverall efficiency.

The suction volumes of the compressors are also minimized. Pipelinedimensions are reduced with smaller valves and actuators as aconsequence. Space need will be considerably reduced and the cost willbe lower. The installation work will also become less complicated andmore efficient.

A major improvement for the energy efficiency is the use of threeseparate expander circuits adapted to the three different stages of thenatural gas liquefaction. This is unlike prior art technology, e.g. inthe U.S. Pat. No. 6,412,302, not having separate adaptation forde-superheating and condensation/cooling of dense phase. Thethermodynamic result of the described system can be seen in FIG. 3. Byadapting the mass flows, pressure ratios and temperatures of eachexpander 1, 2 and 3, the heat exchanger losses indicated by the distancebetween the cold and warm composite curves, can be reduced to a minimum.

The present refrigeration arrangement will operate with the refrigerantin the gas phase. Nitrogen is an obvious gas to apply, since it hasfavourable properties and is a proven refrigerant. The mole weight ishigher than for methane. High molecular weight is advantageous when usedin turbo compressor machinery. Methane or hydrocarbon mixtures areproposed used in the U.S. Pat. No. 6,412,302. Hydrocarbons are alsoflammable, which is regarded as a disadvantage in some applications, forinstance in offshore installations.

FIG. 4 shows a second embodiment in which each of the expanders 1, 2, 3is operated in separate cycles with its own compressor configuration.The expander 1, 2, 3 are supplied from the compressor 13, compressors14, 15, and compressors 16, 17, 18, respectively. The number ofcompressors or compressor stages may vary in each cycle. As beingillustrated in FIG. 3, each of the expanders 1, 2 3 will supply the coldbox 8 with refrigeration capacity adapted to the different temperaturezones.

Separate cycles give improved flexibility with regard to pressure,temperature and mass flow control, i.e. the refrigeration capacity atthe different natural gas liquefaction process stages. Each cycle can becontrolled separately with inventory control and compressor speedcontrol. An example of an inventory control assembly is shown in FIG. 4.The three separate cycles are connected to an inventory buffer vessel25, which is kept at a pressure lower than the lowest high pressure inthe cycles, and higher than the highest low pressure in the cycles. Thevalves 26 to 31 will be used to transfer mass between the cycles and thevessel 25. Even though the cycles work separately, they are connectedand dependent of each other when controlling the arrangement. Separateinventory control gives the possibility to vary the overall pressurelevels in each cycle.

The flexible control philosophy makes the system with separate cyclesrobust and adaptable to variations in gas source flows and compositions,and start up situations. A possible disadvantage may be the need of morecompressors, However, the total suction volume will principally notincrease compared to the system shown in FIG. 3.

Using three expanders in the process of LNG production is basicallyadvantageous as illustrated in FIG. 1. However, even higher efficienciescan be achieved with the use of four expanders or more, not shown. Thereason is an even better adaptation between the warm and cold compositecurve. Increased complexity can probably be accepted in large scaleplants where energy efficiency is decisive.

FIGS. 5 and 6 show embodiments for LNG production based on the sameprinciples as illustrated by FIGS. 3 and 4, but with two expandersinstead of three. FIG. 5 depicts an example having a common compressortrain, and FIG. 6 shows an example comprising separate cycles. In bothof the cases illustrated, the expander 3 is adapted to sub-cooling theliquefied natural gas, whereas the expander 2 is adapted tode-superheating and condensation/cooling of dense gas. The expander 2 ishence used for production of liquefied natural gas, whereas the expander3 is used for sub-cooling. The adaptation between the warm and coldcomposite curves will be poorer compared to the solutions having threeexpanders, but the configuration is less complex. The total compressorsuction volume will not decrease compared to the embodiment having threeexpander, since the suction capacity of the compressors 6, 5 or 14, 15must be increased to handle both de-superheating and condensation/densegas cooling.

As for the described systems with three expanders, the capacity controlcan be performed by inventory control and compressor speed control. Forthe separate cycles, see FIG. 6, pressure levels can be controlledindependently for the two cycles. Inventory control is carried out by arefrigerant mass buffer system including a vessel 25 and the valves 28,29, 30 and 31. Pressure in the vessel 25 is kept lower than the lowesthigh pressure and higher than the highest low pressure in the system.The valves are used for mass transfer to and from the vessel. For theconnected system in FIG. 5, the inventory control is arranged by avessel 25 and the valves 32 and 34. By varying the process inventory,the overall pressure levels can be changed and capacity controlled.Compressor speed variation can be used to vary the overall capacity, butalso for separate control of each compressor stage giving theopportunity to vary capacity on different pressure levels.

The expander 2 in FIGS. 5 and 6 provides the cooling capacity in thehigh temperature cycle. This cooling capacity can for instance beprovided by two expanders in series, see FIG. 8. The mass flow 55 willfirst be expanded in expander 2 a down to an intermediate pressure andsub-cooled in the cold box 8, before a final expansion through a secondexpander 2 b down to the low pressure of the high temperature cycle.Complexity will be slightly increased, but it will improve the energyefficiency. In principle, any of the expanders 1, 2 and 3, can bereplaced by two or more expanders in series.

All the above proposed solutions are not limited to liquefied naturalgas production. Reliquefaction of boil off gas, which also is regardedas a natural gas, is another application wherein the present inventioncan be used, for instance on marine LNG carriers and in onshoreterminals.

Although not illustrated in the drawings, it is understood that morethan three expanders are applicable, e.g. four or even more.

EXAMPLE

Applying the present invention, e.g. as shown in FIG. 3 to a typicalnatural gas source, calculated energy efficiencies of around 0.32 kWh/kgLNG can be achieved, depending on the external conditions. Comparing toprior art solutions, e.g. according to U.S. Pat. No. 6,412,302 which hasa calculated energy efficiency of 0.44 kWh/kg LNG at equal ambientcondition and based on operational data suggested in this description,it is a significant improvement.

The invention claimed is:
 1. A method for producing liquefied andsub-cooled natural gas using a refrigeration assembly comprising asingle phase gaseous refrigerant and comprising at least tworefrigeration cycles each comprising an expander arranged in an expanderloop, wherein a single-component refrigerant is used as the refrigerantin all expander loops, a compressor assembly, a heat exchanger structurefor heat absorption from natural gas, a heat rejection assembly and aninventory vessel, the method comprising: passing an expanded refrigerantflow from each respective expander into the heat exchanger structure,each expander being at a mass flow and temperature level adapted toperform de-superheating, condensation or cooling of dense phase and/orsub-cooling of natural gas; and serving the refrigerant to eachrespective expander in a compressed flow using the compressor assemblyhaving compressors or compressor stages enabling adapted inlet andoutlet pressures for each respective expander, wherein each of therefrigeration cycles is connected to the inventory vessel, andrefrigeration capacities of the refrigeration cycles are controlledindependently from each other by varying refrigerant inventories of therefrigeration cycles using the inventory vessel and valves.
 2. Themethod of claim 1, wherein the refrigerant comprises nitrogen.
 3. Themethod of claim 1, wherein the expanders are connected to the compressorassembly as to fluidly form an integrated refrigeration assembly withseparate expander loops.
 4. The method of claim 1, wherein each expanderis connected to the compressor assembly as to fluidly form separaterefrigeration cycles.
 5. The method of claim 1, wherein refrigerationcapacities are independently varied in each of at least two cycles byseparate inventory control.
 6. The method of claim 5, wherein therefrigeration capacities are controlled by compressor speed control. 7.The method of claim 1, wherein two or more expanders are connected inseries with intermediate cooling between expander stages.
 8. A systemfor producing liquefied and sub-cooled natural gas comprising arefrigeration assembly using a single phase gaseous refrigerant, therefrigeration assembly comprising: at least two refrigeration cycleseach comprising an expander; a compressor assembly; a heat exchangerstructure for heat absorption from natural gas; a heat rejectionassembly; and an inventory vessel, wherein the expanders are arranged inexpander loops, a single-component refrigerant is used as therefrigerant in all loops, an expanded refrigerant flow from eachrespective expander is passed into the heat exchanger structure, eachrespective expander being at a mass flow and temperature level adaptedto perform de-superheating, condensation or cooling of dense phaseand/or sub-cooling of natural gas; the refrigerant to each respectiveexpander is served in a compressed flow using the compressor assemblyhaving compressors or compressor stages enabling adapted inlet andoutlet pressures for each respective expander; each of the refrigerationcycles is connected to the inventory vessel, and refrigerationcapacities of the refrigeration cycles are controlled independently fromeach other by varying refrigerant inventories of the refrigerationcycles using the inventory vessel and valves.
 9. The system according toclaim 8, wherein the refrigerant comprises nitrogen.
 10. The system ofclaim 8, wherein the expanders are connected to the compressor assemblyas to fluidly form an integrated refrigeration assembly with separateexpander loops.
 11. The system of claim 8, wherein each expander isconnected to the compressor assembly as to fluidly form separaterefrigeration cycles.
 12. The system of claim 8, wherein refrigerationcapacities are independently varied in each cycle by separate inventorycontrol.
 13. The system of claim 12, wherein the refrigerationcapacities are controlled by compressor speed control.
 14. The system ofclaim 8, comprising two or more expanders connected in series withintermediate cooling between expander stages.